Most of the conventional photoreceptors for electrophotography use photosensitive layers that comprise inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide. But these photoreceptors do not have a good balance of sensitivity, heat stability, moisture resistance and durability, and in particular, those using selenium and cadmium sulfide have been difficult to manufacture and handle because of the toxicity of these photoconductive materials. Photoreceptors using photosensitive layers that comprise organic photoconductive compounds are gaining acceptance nowadays since they are easy to manufacture and handle, are low in cost and have higher heat stability than those using selenium as a photoconductive material. The best known organic photoconductive compound is poly-N-vinylcarbazole, and a photoreceptor using a photosensitive layer comprising a charge transfer complex made from said compound and a Lewis acid such as 2,4,7-trinitro-9-fluorenone is used commercially, but it is not altogether satisfactory in respect of sensitivity and durability.
Double-layer or dispersion function separated photoreceptors wherein the functions of carrier generation and transport are fulfilled by two separate materials are known, and they have great latitude in selecting components suitable for making products having desired photographic characteristics such as charge characteristics, sensitivity and durability. Various combinations of carrier generation materials and carrier transport materials have been proposed, and a photoreceptor using a photosensitive layer that consists of a carrier generation layer comprising amorphous selenium and a carrier transport layer comprising poly-N-vinylcarbazole is in commercial use. However, the carrier generation layer comprising amorphous selenium does not have high heat stability.
Using organic dyes and pigments as carrier generation materials has been proposed, and photoreceptors containing monoazo compounds or bisazo compounds in photosensitive layers have been disclosed in Japanese Patent Publication No. 30513/73, Japanese Patent Applications (OPI) Nos. 4214/77, 133445/78 and 46558/79 (the symbol OPI as used herein means an unexamined published Japanese patent application) and Japanese Patent Publication No. 11945/81. But the azo compounds used in these patents are not always satisfactory in respect of sensitivity, residual potential or stability to repeated use. What is more, they are compatible with only a limited class of carrier transport materials, and no photoreceptor that completely meets the requirements for electrophotographic process has been found. The bisazo compound disclosed in Japanese Patent Application (OPI) No. 116040/81 is not highly stable and it does not have high sensitivity in the long wavelength range.